22 - Electric Fields

Question 1

Define electric field strength at a point in space.

Answer 1

The force experienced per unit positive charge at that point.

Question 2

What is an electric field caused by?

Answer 2

Charged objects

Question 3

What sort of objects experience force in an electric field?

Answer 3

Charged objects

Question 4

Is electric field strength a vector?

Answer 4

YES

Question 5

What is the direction of an electric field?

Answer 5

The direction that a positive charge would move when placed in the field.

POSITIVE TO NEGATIVE

Question 6

What does the arrow on an electric field line represent?

Answer 6

The direction of the electric field

Question 7

What are electric field lines always perpendicular to?

Answer 7

The surface of a conductor.

Question 8

How is the strength of an electric field represented by field lines?

Answer 8

Their density

Question 9

What is Coulomb’s Law?

In words

Answer 9

Any two point charges exert an electrostatic force on each other that is directly proportional to the product of their charges and inversely proportional to the square of the distances between them.

Question 10

What equation defines Coulomb’s law?

Answer 10

F = kQq / r²

Question 11

What is ε₀?

Answer 11

The permitivity of free space?

Question 12

What do we treat a uniformly charged sphere as?

Answer 12

A point charge

Question 13

How do we calculate the electric field strength of a point charge using Coulomb’s law?

Answer 13

E = kQ / r²

Question 14

What does the electric field strength of a point charge NOT depend on?

Answer 14

The charge of the point charge in the field.

Question 15

What is the main difference between electric and gravitational fields?

Answer 15

Gravitational fields are always attractive whereas electric fields can be either attractive or repulsive.

Question 16

What sort of field does a point charge produce?

Answer 16

A radial field.

Question 17

What is the equation for electric field strength between two parallel plates?

Answer 17

E = V / d

Question 18

What are the two units for electric field strength?

Answer 18

NC⁻¹

Vm⁻¹

Question 19

What two factors does the capacitance of a parallel plate capacitor depend on?

Answer 19

The surface area of the plates, A
The distance between the plates, d

C ∝ A / d

Question 20

What is the constant of proportionality in the relationship, C ∝ A / d?

Answer 20

ε₀ (For air)

Question 21

What do we use instead of the permitivity of free space when a dielectric other than air is used in a parallel plate capacitor?

Answer 21

ε

ε = ε₀ε₁ (Where ε₁ is the permitivity of the dielectric)

Question 22

How would you suspend a droplet of a charged liquid in an electric field?

Answer 22

Make the force of the electric field equal to the weight of the droplet.

Question 23

If an object between two parallel plates experiences a constant force due to the electric field, describe its acceleration?

Answer 23

Constant as well

Question 24

What must we do when calculating anything from a particle travelling HORIZONTALLY in a uniform electric field?

Answer 24

Consider its vertical and horizontal components separately.

Question 25

How do charges store energy?

Answer 25

Two like charges repel each other. By forcing them together we are storing potential energy as we need to do work. This energy is released when we release the charges.

Question 26

Where is the energy represented on a force distance diagram?

Answer 26

The area under the curve

Question 27

How do we calculate the work done on a charge?

Answer 27

W = Fx

Question 28

Define electric potential energy at a point

Answer 28

The work done per unit charge in bringing a point charge from infinity to that point.

Question 29

Why does the equation for electric potential energy only have a single r in the denominator?

Answer 29

Because FORCE has r² in its denominator and work is force x distance so we get rid of one of the rs.

Question 30

What is the unit for electric potential?

Answer 30

JC⁻¹ Volts

Question 31

What is electric potential difference?

Answer 31

The difference in electric potential energy between two points. The work done per unit charge to move a point charge between two points in an electric field.

Question 32

How do we calculate electric potential difference?

Answer 32

Calculate electric potential at both points and subtract them.

Question 33

What is the symbol for electric potential?

Answer 33

V | NOT UNIT

Question 34

What is the capacitance of an isolated sphere, radius R?

Answer 34

C = 4πε₀R

Question 35

What is an isolated sphere?

Answer 35

A sphere that is far away from other objects.

Question 36

What can an isolated sphere act as?

Answer 36

A capacitor.

Question 37

What is the difference between energy and electric potential regarding electric fields?

Answer 37

Energy is not per unit charge. Potential is per unit charge.